A letter written in jail by accused Christchurch mosque gunman Brenton Tarrant has been handed to police for investigation, the Department of Corrections says.

Meanwhile, it's also confirmed another white supremacist has been allowed to send a second letter from prison that should have been stopped.

The department has been in damage control this week after it was revealed Tarrant had managed to send two letters from prison that could have been withheld by prison authorities, including one to a far-right contact in Russia who posted the message online.

A spokeswoman for Corrections on Friday confirmed another letter - one that had been withheld – had been passed on to police for further investigation.

Advertisement

She would not say on what grounds the letter had been referred or what it contained.

Police said they were liaising with Corrections about prison letters, but declined to comment further.

The accused gunman has sent a total nine letters from jail, two which were stopped by prison authorities and seven - including some to his mother - which were released.

In the letter posted online, he wrote about his political and social views but said he could not go into great detail about his regrets or feelings "as the guards will confiscate my letter if I do [to use as evidence]".

Part of the letter included what has been described as a "call to arms".

And the Department on Friday also confirmed more troubling letters from Christchurch white supremacist Philip Arps had been allowed out.

Arps is serving a jail sentence at Christchurch Men's Prison for sharing the mosque shooting video.

TVNZ reports it has received a strongly racist letter from Arps praising the alleged Christchurch gunman.

Advertisement

Newshub also says it has now received two letters from him containing extremist content.

Corrections chief executive Christine Stevenson on Friday apologised again said she no longer had confidence the way prison mail was being handled.

"Late this afternoon, it was brought to my attention that a second letter from a prisoner holding extremist views has been sent out of Christchurch Men's Prison," she said.

"This is totally unacceptable, it should not have happened, and I apologise for any further distress this has caused."

She said she had assured Corrections Minister Kelvin Davis that all mail coming from prisoners identified as having extremist views would be centralised while a review was carried out.

"This has also highlighted the need for us to look at the current legislation and how it is being applied and whether it is fit for purpose," she said.

Sending mail is legal right for prisoners under the current law, but authorities may withhold letters on some grounds.

Davis says Cabinet will discuss the legislation when it meets on Monday.

Corrections has said it's also change its policies on checking mail, including going through a series of teams and experts before getting Stevenson's sign-off.